Nowadays liquid crystal displays (LCDs) are commercially used in almost any area, where information is displayed electronically. High resolution LCDs are for example used in television screens, computer monitors, laptops, tablet PCs, smart phones, mobile phones and digital cameras. Although size and applications are quite different, all of these LCDs are video capable and require high speed switching and high contrast ratio. In order to realize high contrast, it is most important to provide a very low dark state brightness. In “normally white” LCD modes, like a standard TN-LCD, the dark state is achieved by applying a voltage to the LCD. Consequently, the light transmission in the dark state can be controlled by the applied voltage. In case of “normally black” mode LCDs, such as the vertical alignment (VA) mode, in-plane switching (IPS) or fringe field switching (FFS), the dark state corresponds to the non-activated state and therefore the dark state brightness cannot be adjusted by a voltage. Consequently, the dark state mainly depends on the quality of the liquid crystal alignment in the LCD. For VA-mode LCDs the low dark state brightness is achieved if all of the liquid crystal molecules are aligned almost perpendicular to the LCD surface, as then a viewer looking from a direction perpendicular to the screen looks along the optical axis direction of the liquid crystal molecules, in which the liquid crystals do not exhibit birefringence.
In case of planar modes, such as IPS and FFS, the liquid crystal director in the dark state is oriented parallel or perpendicular to the polarization directions of the attached, normally crossed polarization films. Liquid crystal domains, which are not perfectly aligned in the desired direction, introduce birefringence, which causes light leakage due to depolarization of the light. Hence, well defined azimuthal anchoring of the liquid crystals on the alignment layers is crucial to guarantee low dark state brightness for planar mode LCDs, in particular when operated in the normally black mode.
When applying a voltage to an LCD to switch it to a grey or bright state, the liquid crystal layer is deformed and again the alignment layer has to provide strong anchoring forces for the liquid crystals in order to drive them back to the initial off-state configuration, as soon as the applied voltage is below the threshold voltage of the LCD. Any deviation to the initial off-state configuration will be observed as image sticking and therefore display quality is reduced. Because an alternate current (AC) voltage is applied to switch the LCD to different grey levels, the image sticking, which occurs after the AC-voltage is changed or removed is also referred to as AC-memory.
Conventionally, alignment of the liquid crystals in LCD production has been done by brushing a thin polymer layer on the LCD substrates with a cloth. As this process becomes more and more challenging because of the increasing size of the motherglass, there is strong demand for alternative alignment methods.
The most promising approach for replacing the brushing process is photo-alignment. Contrary to brushing, photo-alignment avoids mechanical contact with the surface of the alignment layer. As a consequence, photo-alignment does not create mechanical defects, and hence it offers a very high yield in production.
Photo-alignment has been successfully introduced into mass production of VA-LCDs a few years ago and is now an established technology for LCD alignment. On the other hand, despite the strong demand from LCD manufacturers for photo-alignment of planar mode LCDs, it has not been introduced in the production of such LCDs so far. The reason is that photo-alignment materials have so far not satisfied the challenging alignment quality requirements of planar mode LCDs in terms of display contrast and image sticking.
It is therefore an object of the invention to provide new photo-aligning materials and photo-alignment layers with high anchoring, for planar LCD modes, which enable high contrast LCDs with reduced AC-memory.